MIMO is known as a technique which provides a plurality of antennas on both the transmitting side and the receiving side, provides a plurality of radio wave channels in a space between radio transmission and reception and transmits a signal by spatially multiplexing the channels. It is possible to improve the transmission efficiency by using MIMO.
A link adaptation technique is one of peripheral element techniques of MIMO. Link adaptation is a technique of adaptively controlling an M-ary number (transmission rate) of modulation scheme, coding rate, transmission power distribution, or the like according to fluctuation of channel environments between transmission and reception. When link adaptation is applied to MIMO and encoding is performed in stream (transmitting antenna or beam to which data is allocated) units, MIMO channels can be effectively used. As such a technique, a technique disclosed in Patent Document 1 is known.
FIG. 1 is a block diagram showing the configuration of a MIMO system disclosed in Patent Document 1. This MIMO system assumes a system which performs encoding and transmission power control after dividing a signal into streams, and channel estimator 21 of receiver 20 performs channel estimation, and power/rate calculator 22 determines rates and power of the streams using the channel estimation values. Receiver 20 feeds back indicators indicating the rates and the power determined by power/rate calculator 22 to transmitter 10. Transmitter 10 refers to the indicators fed back from receiver 20 and derives rates and power to be applied to the streams. In this way, it is possible to set transmission rates and transmission power according to channel conditions of the streams and realize high-speed transmission with secured reliability.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-217752